1. Field of the Invention
The present invention includes a system for remotely operating sliding valves of a fracing system for production of fluids, such as oil or natural gas. Sliding valves may be selectively opened or closed according to preference of a well operator.
2. Description of the Related Art
Fracing is a method of stimulating a subterranean formation to increase the production of fluids, such as oil or natural gas. In hydraulic fracing, a fracing fluid is injected through a wellbore into the formation at a pressure and flow rate at least sufficient to overcome the pressure of the reservoir and extend fractures into the formation. The fracing fluid may be one of any number of different media, including, but not limited to, sand and water, bauxite, foam, liquid carbon dioxide, or nitrogen. The fracing fluid keeps the formation from closing back upon itself when the pressure is released. Injecting fracing fluid into the formation provides channels through which the formation fluids, such as oil and gas, can flow into the wellbore and be produced.
Rudimentary fracing methods require cementing a well casing in place and then perforating the well casing at the producing zones, a process that requires packers between the various stages of the producing zone. U.S. Pat. No. 6,446,727 (the '727 patent) shows perforating the well casing to gain access to the producing zone. Perforating the well casing requires setting off an explosive charge in the producing zone, which can many times cause damage to the formation. In addition, once the well casing is perforated, isolating a particular zone becomes difficult, normally requiring the use of packers both above and below the zone.
U.S. Pat. No. 5,894,888 (the '888 patent) also shows an example of producing in the open hole by perforating the well casing. One problem with the '888 patent, however, is that the fracing fluid is delivered over the entire production zone, thus preventing concentrated pressures in preselected areas of the formation. Once the well casing is perforated, it is very difficult to restore and selectively produce certain portions of the zone and not produce other portions of the zone.
When fracing with sand, sand can accumulate and block flow. U.S. Published Application 2004/0050551 (the '551 application) shows fracing through a perforated well casing and the use of shunt tubes to give alternate flow paths. The '551 application, however, does not provide a method for alternately producing from different zones or stages of a formation.
One method used in producing horizontal formations is to provide a well casing in the vertical hole almost to the horizontal zone being produced. At the bottom of the well casing, one or more holes extend horizontally. A liner hanger is set at the bottom of the well casing with production tubing then extending into the open hole. Packers are placed between each stage of production in the open hole, with sliding valves along the production tubing opening or closing depending upon the stage being produced. U.S. Published Application 2003/0121663 shows packers separating different zones to be produced with nozzles (referred to as “burst disks”) placed along the production tubing to inject fracing fluid into the formations. There are, however, disadvantages to this particular method. For one, the fracing fluid will be delivered the entire length of the production tubing between packers. This means there will not be a concentrated high pressure fluid being delivered to a small area of the formation. Also, the packers are expensive to run and set inside of the open hole in the formation.
Published patent applications 2004/0129422, 2004/0118564, and 2003/0127227 show packers used to separate different producing zones. The producing zones, however, may be along long lengths of the production tubing rather than in a concentrated area.
U.S. Pat. No. 7,267,172 to Hofman shows a method and apparatus for overcoming many of the problems associated with fracing. Production tubing and sliding valves are cemented in place in the open hole. When an area is to be fraced, a sliding valve is opened, the cement is dissolved by acid or other dissolvent to allow access to the formation only adjacent to the sliding valve. By selectively opening one or more valves along the production tubing, the well operator can concentrate high pressure fracing fluid to a small area of the formation adjacent the open sliding valve, while the undissolved cement prevents the migration of the fracing fluid to other areas. The high pressure fracing fluid thus penetrates deeper into the formation, facilitating recovery of greater amounts of fluids while using less fracing fluid.
Manual shifting of the sliding valves, however, is both time consuming and cumbersome. A shifting tool must be manually lowered sometimes great distances with a shifting string into the production tubing, engage the desired sliding valve, and then move the sliding valve to the desired position. If a well operator wishes to open multiple sliding valves (or close multiple sliding valves), the process takes even more time, as each sliding valve must be manually manipulated with the shifting tool. The shifting tool must be inserted and removed each time it is used. The process must be repeated when closing one sliding valve and opening another sliding valve, as shown in the Hofman application.
The present invention simplifies and expedites the process of shifting the sliding valves in the producing regions by remotely operating sliding valves.